In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, transmission schemes and reception schemes based on the use of narrow beams might be needed at high frequencies to compensate for propagation losses. For a given communication link, a beam can be applied at both the network side (such as at the transmission and reception point (TRP) of a network node) and the user side (such as at terminal devices served by the network node). A beam pair link (BPL) is defined by the beam used by the TRP (denoted TRP beam) for communicating with the terminal device and the beam used by the terminal device (denoted TD beam) for communicating with the TRP. Each of the TRP beam and the TD beam could be used for any of transmission and reception. Likewise, there could be separate BPLs for downlink communications (where the TRP beam is a transmission (TX) beam and where the TD beam is a reception (RX) beam) and uplink communications (where the TRP beam is an RX beam and where the TD beam is a TX beam).
In general terms, a beam management procedure is used to discover and maintain BPLs. A BPL is expected to be discovered and monitored by the network using measurements on downlink reference signals used for beam management, such as channel state information reference signals (CSI-RS).
The CSI-RS for beam management can be transmitted periodically, semi-persistently or aperiodic (such as being event triggered) and they can be either shared between multiple terminal devices or be device-specific.
In order to find a suitable TRP beam the TRP transmits CSI-RS in different TRP TX beams on which the terminal devices performs reference signal received power (RSRP) measurements and reports back the N best TRP TX beams (where the value of N can be configured by the network). Furthermore, the CSI-RS transmission on a given TRP TX beam can be repeated to allow the terminal device to evaluate suitable TD beams, thus enabling so-called TD RX beam training.
The terminal devices and/or the TRP of the network node could implement beamforming by means of analog beamforming, digital beamforming, or hybrid beamforming. Each implementation has its advantages and disadvantages. A digital beamforming implementation is the most flexible implementation of the three but also the costliest due to the large number of required radio chains and baseband chains. An analog beamforming implementation is the least flexible but cheaper to manufacture due to a reduced number of radio chains and baseband chains compared to the digital beamforming implementation. A hybrid beamforming implementation is a compromise between the analog and the digital beamforming implementations. As the skilled person understands, depending on cost and performance requirements of different terminal devices, different implementations will be needed.
During beam pair establishment (e.g. using the example of TD RX beam training for a terminal device with an analog antenna array), it is expected that the terminal device scans through narrow pencil beams pointing in different directions and then selects the TD RX beam that gives the highest measured RSRP. One reason for using narrow beams is that the narrower the beams, the higher the antenna gain. Such narrow high gain beams are especially useful in line of sight channels where the angular spread in the channel seen by the terminal device is rather small. However, there could be situations when it is more beneficial to use a broad beam, and it could hence be difficult for the terminal device to select which beam to use.
Hence, there is still a need for an improved beam selection procedure.